1. Field of the Invention
This invention generally relates to a solid polymer electrolyte fuel cell stack wherein certain of the cells are more resistant to degradation and, more particularly, to stacks having end cells that are more resistant to degradation during operation.
2. Description of the Prior Art
Significant effort has been directed to the development of reliable fuel cell systems for use as power supplies in a wide variety of applications, such as stationary power plants and portable power units. Such systems offer the promise of economically delivering power while providing environmental benefits.
In general, fuel cells convert fuel and oxidant reactants to generate electric power and reaction products, and often employ an electrolyte disposed between cathode and anode electrodes. A catalyst typically induces the desired electrochemical reactions at the electrodes. A preferred fuel cell type, particularly for portable and motive applications, is the solid polymer electrolyte (SPE) fuel cell, which comprises a solid polymer electrolyte and operates at relatively low temperatures.
SPE fuel cells employ a membrane electrode assembly (MEA) which comprises the solid polymer electrolyte disposed between the cathode and anode electrodes. Each electrode contains a catalyst layer, comprising an appropriate catalyst, located next to the solid polymer electrolyte. The catalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support). The catalyst layers may contain ionomer similar to that used for the solid polymer membrane electrolyte (e.g., Nafione®). The electrodes may also contain a porous, electrically conductive substrate that provides mechanical support, electrical conduction, and/or reactant distribution, thus serving as a fluid diffusion layer. Flow field plates for directing the reactants across one surface of each electrode or electrode substrate, are disposed on each side of the MEA. In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, numerous cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack.
During normal operation of a SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the solid polymer electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant at the cathode catalyst to generate water as a reaction product.
However, other reactions can take place when abnormal conditions exist in a SPE fuel cell. For example, as disclosed in published PCT applications WO 01/15255, WO 01/15249 and WO 01/15254, electrolysis of water and/or corrosion of certain fuel cell components can occur when fuel cells undergo voltage reversal. Fuel starvation conditions can, for instance, lead to a voltage reversal situation in which the anode components are subject to corrosion. Various methods are disclosed in the above published applications to prevent or reduce such corrosion.
Generally, prior art methods have sought to prevent or reduce corrosion of all the fuel cells within the stack. An alternative strategy has been to sense or detect an adverse situation at a point in time such that remedial action can be taken. In published PCT application WO 00/02282, a sensor cell is specially designed to be sensitive to adverse conditions. Early detection is accomplished by monitoring the sensor cell and comparing it to other cells in the stack. Once detected, corrective action can be taken to address the adverse condition. Adverse conditions can also arise in a SPE fuel cell upon startup and shutdown. In published U.S. patent applications US2002/076582 and US2002/076583, conditions are disclosed that can lead to cathode corrosion during startup and shutdown. Such corrosion may be reduced by rapidly purging the anode flow field with an appropriate fluid.
In an SPE fuel cell stack containing a large number of individual cells, the condition of end cells in the stack (i.e., those cell or cells at or near an end of the stack) can be abnormal and different from the rest of the fuel cells within the stack because of differences in operating temperature. Heat loss to the surrounding environment by, for example, convection or conduction can make the end cell or cells cooler than those in the rest of the stack, which in turn can result in poor performance or premature failure of the end cell(s). An example of such a temperature difference is disclosed in published Japanese Patent Application JP08-167424, which also discloses the use of heating elements in the end collector plates of the fuel cell stack to compensate for the heat loss in the end cells. Similarly, published U.S. patent application US2001/0036568 also discloses the use of heatable elements to heat the end plate of a SPE fuel cell stack. In this published application, it is also noted that water may condense in colder end cells, causing flooding and leading to voltage reversal and degradation of materials in the fuel cell. While heating elements can be used to compensate for heat loss from the end cells, thereby allowing them to perform similarly to the rest of the cells in the stack, this approach requires additional hardware and complexity.
While significant advances have been made related to fuel cells generally, and to SPE fuel cell stacks specifically, there remains a need in the art for improved fuel cells, fuel cell stacks and related systems that are more resistant to degradation. The present invention fulfils these and other needs as set forth in greater detail below.